This invention relates to devices for utilizing thermal energy to heat a contained fluid and for the storage of excess thermal energy for maintaining the fluid in a heated condition.
As a result of recent substantial increases in the cost of commonly-used energy sources such as oil, gas and utility-supplied electricity, the search for alternative sources of inexpensive energy has led to expanded interest in the utilization of sunlight for providing power. In the usual instance, a solar collector panel is positioned to receive incident solar radiation thereon and a primary fluid is circulated in the solar collector for heating by the sunlight impinging thereupon. The primary fluid heated in the solar collector may be advantageously employed to heat a secondary material for a desired application. Where the secondary material used is water, solar energy may be conveniently and inexpensively utilized to provide heated water.
Thus, U.S. Pat. No. 4,003,367 to Wikholm discloses a solar water heater in which initially cool water is circulated through a tubular heat exchange coil positioned in thermal communication with a primary fluid heated in a solar collector. Thermal energy as heat is transferred from the primary fluid to the water during the latter's passage through the coil so as to raise the temperature of the water. However, the Wikholm device fails to include provision for the storage of thermal energy in excess of that necessary to heat the primary fluid. Although the heated primary fluid storage chamber of Wikholm is insulated, once solar radiation is no longer incident upon the solar collector, as for example when the sun sets, the primary fluid cools fairly rapidly and the heating of the water must be accomplished by other means employing more expensive energy sources.
Furthermore, the temperature to which the water is heated during its passage through the heat exchange coil of the Wikholm device is critically dependent upon a number of widely variable factors, including the initial temperature of the water, the velocity of the water as it passes through the coil, and the temperature of the heated primary fluid. Without the provision of complex external control devices, the eventual user of the heated water may be surprised to find the same to be scaldingly hot or maddeningly tepid.
Alternatively, the heated primary fluid may be circulated through a heat exchange coil or the like positioned in thermal communication with a tank or similarly enclosed volume of water to heat the water in the tank to a desired temperature. Thermal energy is thus stored in the specific heat of the water. A thermostat sensing the water temperature can be employed to actuate a cutoff valve at the desired temperature and thereby disconnect the source of heated primary fluid from the heat exchanger so as to prevent the water from further absorbing thermal energy and getting hotter. Such a hot water heater is manufactured by Mor-Flo Industries of Cleveland, Ohio, under the trademark SOLARSTREAM.
Although this device enables substantial control of the water temperature, there is again no provision for the storage of substantial amounts of excess available thermal energy to maintain the desired temperature of the water once the primary fluids begins to cool. Since thermal energy is stored only in the specific heat of the water, the amount of available thermal storage at a given temperature may be increased only by increasing the volume of water in the tank. Even so, in the absence of available heated primary fluid for heat exchange, the temperature of the water will soon begin to drop below the desired temperature and an external source of energy, such as electricity, must then be employed to insure a continued supply of hot water.
It is well known in the art to store thermal energy in the specific heat of an external mass of material such as a particulate pebble or rock bed. Such storage materials are generally quite massive and voluminous and, therefore, require a great deal of space atop a reinforced support surface. It has also been recognized that thermal energy may be stored in certain fusible substances or materials as the latent heat of fusion thereof, as typified by the constructions of U.S. Pat. No. 3,356,828 to Furness and No. 3,532,856 to Collins. However, the use of any of these materials or the like for the storage of excess thermal energy not immediately required to heat the water or other secondary fluid requires considerable external control apparatus to prevent the storage material from initially absorbing thermal energy required to first heat the water. Without such external controls, on a cloudy day when little solar radiation is available to heat the primary fluid for thermal energy transfer therefrom, the water may be unable to absorb sufficient heat to raise the temperature of the same to the desired maintenance temperature due to the simultaneous absorption of much of the available thermal energy by the additional storage material.